sharepoint 2013 Show Errors (why it's not working for you)

To disable the custom error page in  sharepoint 2013 you have to set  
<customErrors mode="On" /> to <customErrors mode="Off" /> 

Ok but this may not work , right ? I tell you why.

For sharepoint there is many web.config files located under:

"C:\inetpub\wwwroot\wss\VirtualDirectories\80"

but non of them will work as expected for pages using layouts.

To make it work, you must modify the web.config under

"C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\LAYOUTS"


Hope it works for you either.